VAN NUYS ( — A surge in whooping cough cases reported in California two years ago may be linked in part to parents’ refusal to vaccinate their children, according to a new study.

KNX 1070’s Margaret Carrero reports the findings come amid new data from health officials that shows the California outbreak of pertussis in 2010 was among the biggest in six decades.

A study entitled “Nonmedical Vaccine Exemptions and Pertussis in California, 2010” published in the medical journal Pediatrics on Oct. 1 found a statistically significant correlation between areas with a high concentration of pertussis cases and areas where higher numbers of parents sought legal exemptions to opt out their children from vaccination.

Study co-author Dr. Daniel Salmon with the John Hopkins-Bloomberg School of Public Health said the decision not to vaccinate occurred most frequently among families considered to be among higher socioeconomic status.

“They’re parents who have claimed a conscientious objection to the school immunization requirements,” Salmon said. “Broadly, we find that parents that don’t vaccinate tend to be higher income, higher education.”

California is one of 19 states in the U.S. that offers both religious and philosophical exemptions from school immunization requirements.

In 2010, ten infant deaths were reported among over 9,100 cases in California, making it the largest such outbreak since 1947.

Salmon along with the study’s authors also underscored the importance of “herd immunity” to ensure any vaccination effort is successful.

“With diseases like pertussis and measles, which have the highest known infectious disease reproduction numbers, it is estimated that 95 percent of the population must be immune to prevent outbreaks and suppress sustained transmission,” according to the study. “Herd immunity must be maintained to reduce the risk of disease for those too young to be vaccinated or unable to receive vaccines.”

The Centers for Disease Control lists infants who are too young to be immunized as being at greatest risk for life-threatening cases of the disease.

Read the full study: “Nonmedical Vaccine Exemptions and Pertussis in California”